1. Field of the Invention
This invention relates to an intake system for an internal combustion engine having a supercharger.
2. Description of Related Art
In recent years, a number of different considerable types of internal combustion engines have been provided with various types of superchargers to supercharge intake air.
In these types of superchargers, their supercharging pressures to be imposed on the intake air are generally limited in their ability to ehnace the intake air pressure by some limiting factors determined by engine characteristics. Taking the upper limits of the supercharging pressure into consideration in relation to engine speeds, in a low engine speed range, the supercharging pressures are to be limited in order to prevent knocking of the engine. In a middle engine speed range, the supercharging pressures are to be limited to assure the presence of appropriate combustion pressures. In a high engine speed range, the supercharging pressures are to be limited for to ensure the presece of appropriate heat loads thereof. In permissible supercharging pressures thus defined by these limiting factors, the permissible supercharging pressures depending on heat load of the engine are, in general, in the lowest level. Therefore, in conventional engines, the maximum supercharging pressures have been substantially selected on the basis of the permissible supercharging pressure depending on the heat loads of the engines.
In internal combustion engines with superchargers as set forth above, it has been proposed that intake systems of the engines be arranged to supercharge the intake air further with a so-called kinetic supercharging effect of intake air (See, for example, Japanese Utility-Model Laid-Open Publication No.62-49625). The kinetic effect is intended to improve the volumetric efficiency of cylinders of the engine so as to enhance the torque of the engine. Such a kinetic effect includes both an inertia effect of intake air and a resonance effect of intake air .
Since a preferred nature of the kinetic effect is obtained in synchronism with a certain engine speed, to obtain take the kinetic effect in an intake system, it has been proposed to provide a means for allowing the intake system to correspond to different engine speeds, thereby providing the advantageous kinetic effect in a wide range of the engine speeds.
For example, U.S. Pat. No. 4,829,941, issued on May 16, 1989 to the same assignee as the present application, discloses one type of intake system having means for changing over the length of the resonance air column in the intake system on the basis of the engine speeds to obtain an advantageous intake air resonance effect over a wide range of engine speeds. Further, the U.S. Pat. No. 4,617,897, issued on Oct. 21, 1986 to the same assignee as the present application, discloses another type of intake system which is adapted to change the length and the cross-sectional area of the intake passage to obtain a preferred intake air inertia effect over a wide range of engine speeds. Furthermore, U.S. Pat. No. 4,651,684 issued on May 24, 1987 to the same assignee as the present application, discloses still another type of intake system in which the valve timing of the intake valve is changed, so as to vary the opening time thereof according to the operating condition of the engine, so that the volumetric efficiency is improved.
In the conventional engines with superchargers such as those previously mentioned, since the maximum value of the supercharging pressure provided by the supercharger is determined on the basis of a limiting factor which depends on the heat load of the engine, the engine can afford to enhance the supercharging pressures in low and middle engine speed ranges. Therefore, it is taken into consideration that the maximum supercharging pressure is shifted in accordance with a parameter defined by engine speeds. In other words, when, in simplification, the engine speed range is supposed to be divided into a relatively low range and a relatively high range, the maximum supercharging pressure is set to a high value in the low engine speed range and the maximum supercharging pressure is set to a low value in the high engine speed range, thereby changing the maximum supercharging pressure over between low and high values in accordance with the engine speeds.
However, in such an arrangement, at the time the maximum supercharging pressure is shifted, there is generated a great fluctuation in engine torque, that is, a torque shock, and, therefore, consideration of this problem has been required.